1. Field of a the Invention
The present invention relates to a method and an apparatus for detecting a spectrum spread signal of an additional information signal which is spectrum spread and superimposed to an information signal such as a video signal.
2. Description of the Related Art
A digital information recording apparatus such as a digital VTR and MD(Minidisc) recording and reproducing apparatus is now widely spreading and moreover a DVD (Digital Video Disk or Digital Versatile Disk) having the recording function has also be developed. In these digital information recording apparatuses, various additional information signals can be recorded in relation to the digital video signal, digital audio signal and data of computer as the main information signal.
In this case, an additional information signal is a digital signal and it is added to the digital information signal as the signal to be recorded to the area which is discriminated regionally from the digital information signal such as the header area, for example, which as added to the data in unit of block of the digital information signal and the other TOC (Table of Contents) area, etc.
As explained above, in the additional information signal recording system of the related art, the additional information signal is not superimposed in direct to the digital information signal such as a header area but superimposed to an indirect area. Therefore, the additional information signal can rather easily be removed by the filtering or alteration and it is sometimes impossible to detect the necessary additional information signal with a recording apparatus or reproducing apparatus. Particularly when a control information or copy right information to prevent unfair duplication is added as the additional information signal, since such additional information signal is removed, the predetermined object cannot be attained in some cases.
Moreover, when the additional information signal is added to the indirect area as explained above, if the digital information signal is converted to an analog signal, only the main information signal can be obtained and thereby the additional information signal is lost. It means that even when a measure to prevent unfair duplication of the digital information signal is provided by superimposing the above-mentioned duplication preventing control signal is superimposed as the additional information signal, if the additional information signal is converted to an analog signal, the means for preventing duplication can no longer be operated effectively.
As a method of superimposing an additional information signal to solve a problem of the additional information to be lost and a problem in conversion to analog signal, the applicant of the present invention has proposed a system wherein the video signal is recorded as the digital signal or analog signal by spectrum spreading the additional information signal such as a duplication preventing control signal and then superimposing this spectrum spread additional information signal to the video signal (U.S. patent application Ser. No. 08/75510).
In this system, the PN (Pseudorandom Noise) series code (hereinafter referred to as PN code) to be used as the spread code is generated in a higher period and this code is then multiplied with the additional information signal for the purpose of spectrum spread. Thereby, the additional information signal such as a narrow band and high level duplication preventing control signal is converted to a broad band and very low level signal. This spectrum spread additional information signal, namely the spectrum spread signal is superimposed to an analog video signal and is then recorded to a recording medium. In this case, the video signal to be recorded to the recording medium may be analog signal or digital signal.
In this system, since the additional information signal such as the duplication preventing control signal is superimposed to the video signal as the spectrum spread broad band and very low level signal, it is difficult, for example, for a person who is trying to execute unfair duplication to remove the superimposed duplication preventing control signal from the video signal.
Meanwhile, it is possible to detect and utilize the additional information signal such as the superimposed duplication preventing control signal by executing the inverse spectrum spread. Therefore, for example, the duplication preventing control signal can surely be provided to the recording apparatus side together with the video signal and this duplication preventing control signal is detected, in the recording apparatus side and the duplication control can surely be realized depending on the detected duplication preventing control signal.
Since the additional information signal superimposed to the information signal such as a video signal is not removed from the main information signal at the time of reproducing the information signal, the spectrum spread signal must be superimposed, as is explained above, to the main information signal in such a low level as not giving influence on the reproduced output of the information signal. In order to superimpose the spectrum spread signal to the main information signal in a very small level, the spectrum spread signal must have sufficiently large spread code length.
Meanwhile, when the inverse spread is requested to detect the spectrum spread signal, a spread code synchronized with the spread code in the spectrum spread process is generated and this spread code for inverse spread is multiplied with the main information signal to which the spectrum spread signal is superimposed in order to detect the additional information signal.
In this case, the synchronous detection of the spread code for inverse spread has been conducted, in the related art, by the method of using the matched filter and by the sliding correlation method, but when the spread code length is large, as explained above, detection becomes impossible or a longer time is taken for the detection. Moreover, since the multiplication is required for inverse spread after the synchronous detection of the spread code for inverse spread, here is also a problem that a longer time is required.
When a longer time is taken for detection of the spectrum spread signal, the duplication control of the video signal is executed, for example, after a considerable time has passed from the start of the reproducing operation of the video signal and thereby the effective duplication control can no longer be executed.
Considering the background explained previously, it is therefore an object of the present invention to provide a method and an apparatus to quickly detect with the simplified structure the spectrum spread signal superimposed to the information signal.
The present invention proposes a method and an apparatus for detecting an additional information signal to produce the spectrum spread additional information signal through the spectrum spread of additional digital information signal using the spectrum spread code produced in synchronization with the synchronous signal included in the main information signal and to detect the additional digital information signal from the main information signal with the additional information produced by superimposing the spectrum spread additional information signal to the main information signal, wherein the spectrum spread code same as the spectrum spread code synchronized with the synchronous signal included in the main information signal with the additional information signal is produced, a value of the main information signal with the additional information corresponding to each chip of the spectrum spread signal is obtained as the accumulated value through addition and subtraction depending on each chip value and the additional digital information signal in the main information signal with the additional information is detected by determining whether the accumulated value has exceeded the positive or negative threshold value.
According to the present invention, when the information signal is, for example, a video signal, since the spread code is synchronized with the video synchronous signal, synchronous generation control of the spread code for inverse spread is basically not required. At the time of detection of spectrum spread signal, the value of the video signal is added, for example, when the chip value is xe2x80x9c0xe2x80x9d or subtracted when the chip value is xe2x80x9c1xe2x80x9d for each chip of the spread code synchronized with the video synchronous signal. The spread code is a random number and the number of chips of xe2x80x9c0xe2x80x9d is equal to the number of chips of xe2x80x9c1xe2x80x9d.
Since the video signal is basically not correlated with the spread code, the addition or subtraction result value in regard to the video signal itself tends to be converged to zero through the addition or subtraction as explained above. On the other hand, since the level of video signal is increased or decreased depending on the chip value xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d, the addition or subtraction result value regarding the spectrum spread signal is routinely increased or decreased. Accordingly, the spectrum spread signal superimposed on the video signal can be detected by previously determining the predetermined threshold value to the addition and subtraction result value and then detecting the level exceeding the threshold value.
In this case, since it is not required that the spread signal is multiplied with the information signal to which the spectrum spread signal is superimposed and the inverse spread is conducted, the structure can be simplified and the detection of the spectrum spread signal can be realized by the software.
When the spectrum spread signal is superimposed to the video signal, for example, in such a manner that level addition is conducted to the video signal when a chip value is, for example, xe2x80x9c0xe2x80x9d or level subtraction is conducted to the video signal when a chip value is xe2x80x9c1xe2x80x9d, when the additional information bit is xe2x80x9c0xe2x80x9d, the level addition and subtraction is conducted to the video signal depending on the chip value of the spread code. On the other hand, when the additional information bit is xe2x80x9c1xe2x80x9d, level addition or subtraction is conducted, for the purpose of superimposing, for the video signal depending on the value of chip in which the spread code is inverted. Therefore, when the addition and subtraction result value exceeds the positive threshold value, the additional information bit can be detected as xe2x80x9c0xe2x80x9d and when the addition and subtraction result value exceeds the negative threshold value, the additional information bit can be detected as xe2x80x9c1xe2x80x9d.